Multiple
Discharges of Treated Municipal Wastewater
Have a Small Effect on the Quantities of Numerous Antibiotic Resistance
Determinants in the Upper Mississippi River

This study evaluated multiple discharges
of treated wastewater
on the quantities of antibiotic resistance genes (ARGs) in the Upper
Mississippi River. Surface water and treated wastewater samples were
collected along the Mississippi River during three different periods
of 4 days during the summer of 2012, and quantitative real-time PCR
(qPCR) was used to enumerate several ARGs and related targets. Even
though the wastewater effluents contained 75- to 831-fold higher levels
of ARGs than the river water, the quantities of ARGs in the Mississippi
River did not increase with downstream distance. Plasmids from the
incompatibility group A/C were detected at low levels in the wastewater
effluents but not in the river water; synthetic DNA containing an
ampicillin resistance gene (<i>bla</i>) from cloning vectors
was not detected in either the wastewater effluent or river samples.
A simple 1D model suggested that the primary reason for the small
impact of the wastewater discharges on ARG levels was the large flow
rate of the Mississippi River compared to that of the wastewater discharges.
Furthermore, this model generally overpredicted the ARG levels in
the Mississippi River, suggesting that substantial loss mechanisms
(e.g., decay or deposition) were occurring in the river.